In optical networks, data from a source may be converted into an optical signal for transmission along an optical fibre. Complex optical transmission devices incorporating lasers for data transmission have been designed and packaged in single units, or modules. However, the signal wavelength outputted from these devices is predetermined by the lasing wavelength of the laser used in the device to generate the optical signal.
Often, it would be desirable to be able to set the output wavelength of the module during installation, and to be able to change it as required, whether for the purpose of colour choice, for compatibility with another optical device or system, or for other purposes.
One previous method was to manufacture a number of different fixed wavelength sourced optical transmission devices. Using the same device design, a variety of units are fabricated, each outfitted with lasers of varying wavelengths This results in expenditures associated with maintaining large inventories of multiple versions of the same device, not all of which may be used, and which may require complex processes to change the output wavelength of an installed source.
Another previous method was the tunable laser. For example, U.S. Pat. No. 5,949,801, issued Sep. 7, 1999 to Tayebati for Tunable Laser and Method for Operating the Same, discloses a tunable Fabry-Perot laser having a tunable Fabry-Perot filter as a wavelength-selective component. The application of a voltage to the filter changes its wavelength-selection properties. However, added complexity in the design and fabrication of tunable lasers increases the costs of such lasers, and hence, the costs associated with the device incorporating a tunable laser. Further, it is difficult to manufacture tunable lasers that are tunable, directly modulated and high speed.
A third method utilizes an attachable component incorporating fibre gratings, or other such component, of a period selected for a particular emitted frequency, that feed back an optical emission into a device with gain and a single mirror to form the oscillator cavity. For example, U.S. Pat. No. 5,978,400, issued Nov. 2, 1999 to Campbell et al., for Laser, discloses a laser diode coupled to an optical fibre having a grating. The laser characteristics depend on the optical phase relationship of the Bragg gratings in the external waveguide. While reducing the complexity incorporated into the optical transmission device, phase shift induced noise is increased as a result of the use of the attachable component. Also, the length of the laser cavity in such a configuration tends to be long where the spectral mode spacing is short. As a result, the likelihood of mode-hopping is increased.
The present invention seeks to provide a method of generating a modulated optical signal and an optical wavelength converter module, which minimizes the above problems.
According to one aspect of the invention, there is provided a method of optical wavelength conversion including the steps of receiving a modulated optical signal of a first wavelength; stimulating an active medium using the modulated optical signal of the first wavelength for generating a like modulated optical signal of a second wavelength; and transmitting the modulated optical signal of the second wavelength; whereby wavelength conversion and modulation are affected entirely within an optical domain.
Also according to the invention, there is further provided an optical wavelength converter including a first part for receiving a modulated optical signal of a first wavelength; a part for stimulating an active medium using the modulated optical signal of the first wavelength for generating a like modulated optical signal of a second wavelength; and a second part for transmitting the modulated optical signal of the second wavelength; whereby wavelength conversion and modulation are affected entirely within an optical domain.
In one aspect of the invention, the first part, the active region, and the second part forms a resonator. The resonator may be of a vertical cavity surface emitting laser (VCSEL) type geometry, edge emitting geometry, optically pumped waveguide geometry, and the like.
In another aspect, the active region is contained in a module. The module is coupled to a source for generating a modulated optical signal of the first wavelength. The coupling is detachable and may be in the form of a detachable pigtail.
In yet another aspect of the invention, the optical signal transmitted by the second part is of a wavelength signal region suitable for optical communications, including amplified systems, wavelength division multiplexing (WDM) systems, and the like.
In yet a further aspect of the invention, the signal transmitted by the second part is along a same directional course as, or a different directional course than, the signal received by the first part, including at an angle to reduce back reflection of the signal received by the first part, in a direction perpendicular to the signal, or the like.
Advantageously, wavelength selection can be accomplished in a relatively simple and inexpensive fashion by providing the wavelength conversion of a fixed wavelength source comprising a simple resonator in an external module. By an optically pumped resonator with its own spectral and spatial filtering functions, and no electrical connections, there is no sensitivity to phase shift-induced noise resulting from use. Also, the configuration can result in oscillation in a mode best coupled to the optical fibre.